1. Field of the Invention
The present invention relates generally to a handoff method in a cellular mobile communication system and a system supporting the same, and in particular, to a method and system for determining handoff, considering a level of an uplink signal.
2. Description of the Related Art
In general, due to the ease of portability of a mobile station (MS) and the frequency with which MS users travel, MSs tend to regularly move between service coverage areas. In this case, a mobile communication system supports handoff in order to seamlessly provide voice or data service. The handoff takes more than only the positional movement of the MS into consideration. For example, a variation in the communication environment and the increasing number of MSs located in the service coverage can also be considered.
The handoff refers to a procedure for efficiently detecting the variation in the communication environment and setting up a new communication path to the optimum base station (BS). That is, the handoff involves forming a communication path to a neighbor BS, if the neighbor BS has a communication environment superior to that of a serving BS. The serving BS refers to a BS from which the MS is receiving a service, and the neighbor BS refers to a BS to which the MS located in the serving BS can easily move. A target BS indicates a neighbor BS to which the MS desires to move.
FIG. 1 illustrates a configuration of a general cellular mobile communication system, wherein each cell is equivalent to a service coverage area.
Referring to FIG. 1, a first MS 112 to a third MS 116 are located in a first service coverage area formed by a first BS 110. Of the MSs, the third MS 116 is located in the region (handoff region) where there is a need for handoff from the first BS 110 to a second BS 120. The first BS 110 is a serving BS of the third MS 116, and the second BS 120 is a target BS of the third MS 116.
Commonly, a handoff method can be classified as follows.
First, the handoff method can be a Break-Before-Make (BBM) handoff scheme or a Make-Before-Break (MBB) handoff scheme according to a disconnect time with the serving BS. The BBM handoff scheme sets up a connection to the target BS after releasing a connection to the serving BS in response to a handoff request. The MBB handoff scheme holds the connection to the serving BS until it sets up a connection to the target BS. The MBB handoff scheme can prevent a ping-pong effect due to the handoff, and obtain spatial diversity gain. The MS releases the connection to the serving BS after it completely leaves the handoff region.
Second, the handoff method can be an MS-initiated handoff scheme, a network-initiated handoff scheme or an MS-assisted handoff scheme according to the subject of performing the handoff. That is, the handoff method can be classified according to the subject of detecting an occurrence of the handoff and determining the target BS.
Finally, the handoff method can be classified into an MS-initialized handoff scheme or a BS-initialized handoff scheme according to a method of initializing measurement of the communication environment for determining whether to perform a handoff.
FIG. 2 illustrates operations of MS-initialized and MS-assisted handoff schemes, which are examples of the aforementioned handoff schemes.
Referring to FIG. 2, an MS connected to a serving BS (serving cell or serving sector) periodically monitors the wireless channel environment for a downlink to a neighbor BS (neighbor cell or neighbor sector) in steps 210, 212 and 214. The MS measures strengths of the signals received from neighbor BSs through the monitoring. Commonly, a pilot signal is used for the monitoring. If the measured signal strength is greater than or equal to a threshold, the MS sends a handoff request to the serving BS. The MS reports an identifier of the signal-measured neighbor BS and the measured signal strength to the serving BS.
The serving BS determines whether to perform handoff depending on the signal strength reported from the MS. The serving BS reports the determination to an upper layer, and sends a handoff command to the MS.
The handoff scheme based on the wireless channel environment of the downlink is used for handoff between BSs using the same frequency band. That is, this handoff scheme is mainly applied to the mobile communication system that acquires code synchronization using searchers and fingers. This is because the MS can measure strength of a signal received from a neighbor BS using a searcher in an idle state even while in data communication with the serving BS. Therefore, there is no need to set up an additional device or a separate search interval. The search interval is used for measuring strength of a signal received from a neighbor BS.
However, in the Broadband Wireless Access (BWA) system in which neighbor BSs use different frequencies or multiple carriers, the search interval should be separately set up. The search interval is set up in the interval where data exchange with the serving BS is not performed. The MS performs an operation of searching for the wireless channel environment of a signal and a downlink, and periodically reports the search result through uplink resource allocation for the serving BS.
As described above, the separate search interval should be set up to apply the existing handoff scheme to the BWA system. In addition, the MS should be allocated the uplink resource used for reporting the search result.